A conventional traction type elevator includes a cab mounted in a car frame, a counterweight attached to the car frame via a rope, and a machine driving a traction sheave that is engaged with the rope. As the machine turns the sheave, friction forces between the grooved surface of the sheave and the rope move the rope and thereby cause the car frame and counterweight to raise and lower. In some applications, liners are disposed in the grooves to improve the traction between the rope and sheave and to minimize wear of the sheave and rope.
The ropes used in elevator applications have traditionally been steel wire ropes. Such ropes are inexpensive and durable. In addition, steel wire ropes tend to be flame retardant. A limiting factor in the use of steel wire ropes, however, is their weight. The higher the rise of the building or hoistway, the longer and heavier the rope becomes. The rope gradually begins to dominate the load to be carried by the elevator system until the weight of the rope exceeds the tensile strength of the rope itself. Another disadvantage is the lubrication required for steel wire ropes. The steel wire ropes are treated with an oil lubrication that ultimately becomes deposited on the hoistway equipment, in the machine room, and in the pit of the hoistway.
There has recently been much interest in replacing the traditional steel wire ropes used in elevator applications with ropes formed from high strength, lightweight synthetic materials, such as aromatic polyamid or aramid materials. Lightweight ropes formed from these materials could potentially reduce the size of many elevator components, such as machines and brakes, and could extend the rise of elevators.
The use of such synthetic ropes in traction elevators poses many problems. First, the ropes will be heavily loaded as they travel over the traction sheave. With conventional sheaves, this will introduce compressive stress onto the ropes and also cause movement of the strands of the rope relative to each other. Typical aramid materials, such as KEVLAR, have a high tensile strength but are more limited in their strength in compression. In addition, rubbing of adjacent strands causes significant abrasion of the materials and quickly degrades the strand fibers.
One proposed solution to prevent damaging abrasion from occurring is disclosed in U.S. Pat. No. 4,022,010, entitled "High-Strength Rope" and issued to Gladenbeck et al. The synthetic rope disclosed in this patent includes a sheath around either the strands or the entire rope. The sheath is formed from a synthetic plastic material, such as polyurethane, polyamide or silicone rubber and its purpose is to provide wear resistance for the strands. A similar solution is proposed in U.S. Pat. No. 4,624,097, entitled "Rope" and issued to Wilcox. A drawback to these solutions is that while permitting relative movement of the strands without abrading, this solution is not optimal for traction.
Another proposed solution is disclosed in Canadian Patent Application No. 2,142,072, entitled "Cable as Suspension Means for Lifts". The rope disclosed in this patent application includes an outer sheath that is extruded onto the outer strands to retain these strands in place while at the same time providing the necessary friction with the traction sheave. Preventing the strands from moving relative to each other, however, may introduce undesirable compressive stresses in the rope as it travels over the traction sheave and thereby limit its durability.
The above art notwithstanding, scientists and engineers under the direction of Applicant's Assignee are working to develop high strength, lightweight ropes formed from synthetic, non-metallic materials that are both effective and durable.